Fiberglass reinforced resinous composites and laminates are well known materials used in many applications, including oil field piping as well as automotive, marine and building applications. Articles are generally prepared by embedding continuous or chopped fiberglass filaments or bundles in a curable resinous binder such as a polyester or epoxy resin, followed by shaping and curing the shaped structure. The fiberglass serves to reinforce the structure giving rise to articles having improved strength and stress and shear resistance.
In order to maximize mechanical properties as well as chemical resistance of the structure, it is necessary that a strong bond be developed between the resin matrix and the fiberglass reinforcement surfaces at their interface. The development of this strong bond may be achieved by coating the fiberglass with a hydrolyzed organofunctional silane coupling agent having the general formula (X).sub.3 Si-R wherein X is a hydrolyzable halogen or alkoxy group and R is an organic radical having functionality which is either capable of reacting with the curable matrix resin or at least highly compatible with the matrix resin. The hydrolyzed silane is initially adsorbed onto the surface of the glass and forms hydrogen bonds with free hydroxyl groups normally present on the glass surface. Subsequent heating of the coated glass surface converts these bonds to covalent siloxane bonds in accordance with the following reaction schematic. ##STR1## Accordingly, it is evident that the quantity of hydrolyzed silane which can be sorbed onto and bonded to the glass surface is a function of the number density of hydroxyl groups available on the glass surface with which the hydrolyzed silane can hydrogen bond.
The present invention provides a method for increasing the number density of hydroxyl groups on glass surfaces by subjecting such surfaces to a water vapor plasma treatment.
It is known in the prior art to subject various substrates to a plasma gas treatment to alter the surface characteristics of the substrate. For example, B. Das discloses in Sample J, 28 (2), 1992 at pages 33-39 the cold plasma treatment of pre-sized glass fiber bundles in the presence of an activated gas such as argon, oxygen, ammonia, Freon.TM. or a mixture of oxygen and Freon. The authors note that some sort of surface modification of the sized fiber surface is achieved as evidenced by differences in water wettability of the treated fibers vs. the untreated fibers. Similarly, V. Krishnamurthy et al. disclose in Journal of Mats Sci 24 (1989) at pages 3345-3352 the argon gas plasma treatment of glass fibers to etch their surface and increase the surface wettability, followed by plasma polymerization onto the treated surface of selected monomers designed to enhance the adhesion of the treated fibers to polymer matrices.
U.S. Pat. No. 4,675,205 discloses the surface treatment of a material, such as an epoxy/fiberglass composite, by subjecting it to a hot gaseous plasma which also may contain a reactive gas such as water vapor or BCl.sub.3. The treatment is disclosed to enhance the adhesion of ceramic materials subsequently coated on the plasma treated surfaces.
However, none of these references disclose exposure of clean glass or fiberglass surfaces to a water vapor-containing plasma and the resulting generation of an increased number density of hydroxyl groups on such surfaces.
It is also known in the art to subject various substrates to glow discharge (plasma) in order to clean or sterilize such surfaces. Such cleaning is required to prepare strongly adhering films via vacuum evaporation and is commonly used in the coating and electronics industry. Glow discharge is also used in the medical field for the cleaning and sterilization of microscope slides and surgical instruments.